The present invention relates generally to flexible circuits and particularly to a flexible circuit termination for high temperature application. Flexible tapes have been used in the past as a lower cost and higher reliability replacement for wiring as an electronic interconnection means. Flexible tapes are routinely populated with electronic components and turned into flexible circuits, which can be die cut, folded and twisted to fit irregularly shaped hardware envelope extending the functionality of stiff glass/epoxy printed circuit board paradigms. Flexible tape and flexible circuits, like printed circuit boards, can be manufactured with multiple layers of copper conductors sandwiched between insulating layers of polyimide. The adhesives holding the layers together are typically acrylic based. Components and terminations on the flexible tapes and boards are typically made with solders through a solder wave or vapor phase process.
Industry has recently developed high temperature solid state integrated electronics and pressure sensing elements amenable to modern Silicon wafer manufacturing economies of scale. These devices are capable of operating at temperatures of 300.degree. C. and above. High temperature applications include, for example, engine testing and oil well logging. In order to implement these high temperature product advances the packaging temperature range for sensors and transducers must be extended upward. The current low cost practical limit of packaging is approximately 185.degree. C. A particularly weak packaging area is the electrical interconnection methodology. The transducer bodies for high temperature applications will take on various form factors in order to satisfy the requirements of different customers. The flexible nature of low cost flexible circuits offers advantages in that a populated flexible circuit could fit into differently shaped transducer bodies.
The flexible circuit prior art indicates photolithographically patterned copper, usually plated with solders of various compositions on polyimide, e.g., DUPONT KAPTON. The polyimide functions as the thin, flexible circuit "board" or substrate. Typically the copper is sandwiched between two layers of polyimide. Flexible circuit complexity can be achieved by increasing the number of copper/polyimide layers and using metal plated vias to connect one layer to another in ways similar to conventional printed circuit boards. The layers are held together by acrylic adhesives. The acrylic adhesives break down at the temperature range of interest obviating their use in these high temperature applications.
The prior art indicates through-hole or surface mounted passive and active electronic components. Through-hole components are typically wave soldered and surface mounted components are usually vapor-phase soldered. Hand soldering could be used, but is often undesirable due to the added labor. The prior art indicates thick copper laminations that can be patterned and selectively etched to form shallow three-dimensional (3-D) features useful in placing and aligning the component. The prior art also indicated two-dimensional (2D) features that can be bent to form 3-D guides or retainers for subsequently soldered component terminations.
The flexible circuit technology of the past is not acceptable for the high temperature applications described herein. The acrylic adhesive breaks down causing delamination of the flexible tape layers. In addition, the high temperature solders are not amenable to high temperature assembly processing or exhibit inherently poor reliability.
Thus a need exists for a flexible circuit termination means which has the inherent advantage of the flexible tape used at low temperature and that can be used at temperatures of 300.degree. C. The advantages include ease of use, manufacturability, low cost and high reliability.